A shoelace fits an upper to the foot. The upper, fitted to the foot, supports the foot.
However, when the MP joint is dorsiflexed, the shape of the foot changes, thereby also changing the foot circumference. When a shoe is worn over a long time, the foot circumference often increases. In such a case, the foot in the shoe will be compressed by the upper. Also, slippage often occurs between the shoe and the foot during movement.
Particularly, the middle foot section of the foot substantially changes its shape as the posture changes. If the upper, which is fitted to the foot when standing still, fails to follow changes of the shape of the foot, the fit between the upper and the foot significantly deteriorates during movement.